The present invention relates to a computer system and control technique for same, and in particular to a technique for performing remote copying in a computer system that comprises a plurality of storage subsystems.
In recent years, in computer systems that comprise a host computer and a storage subsystem containing a storage area for storing data, a strong need to improve reliability of data has arisen. It has become common practice to use remote copy technology in order to improve data reliability in such computer systems. Remote copy technology is a technology wherein data is copied among a number of storage subsystems situated at different sites. According to remote copy technology, in the event that a malfunction occurs in a copy source storage subsystem (primary storage subsystem), rendering it inoperable, it is nevertheless possible to recover the data using data from a copy destination storage subsystem (secondary storage subsystem).
In the meanwhile, processes employing transactions are commonly carried out in computer systems. A transaction refers to a number of related processes that are treated as a single processing unit, such as a deposit/withdrawal process at a financial institution, for example. In a process employing a transaction, it is imperative to avoid situations in which an intermediate result of a transaction is left uncompleted, such as where a deposit process is carried out, but no corresponding withdrawal process is carried out, for example. In order to prevent intermediate results of transactions being left uncompleted, it is necessary for storage areas in storage subsystems to maintain states in which, of data write requests issued by the host computer, those write requests whose issue time is at or prior to a criterion in time are all reflected in storage areas, while those write requests whose issue time occurs subsequent to the criterion in time are all not reflected in storage areas. Here, reflection of a write request in a storage area refers to write data contained in the write request being stored in the storage area.
In computer systems that carry out remote copying, there is known a technique of controlling degradation in performance due to larger distances among storage subsystems, while at the same time preventing intermediate results of transactions from being left uncompleted (for example,JP11-85408A). According to this technique, a primary storage subsystem having received a data write request on which a issue time is stamped from the host computer sends completion information to the host computer immediately after receiving the write request. It is therefore possible to control degradation in performance due to larger distances among storage subsystems. The primary storage subsystem also sends the received write requests to secondary storage subsystems in the order of issue time, whereupon the secondary storage subsystems having received the transmitted write request reflect the write requests in their storage areas, in the order of the issue time stamp appended to write requests. Thus, in the storage areas of secondary storage subsystems, it is possible to maintain states in which, of data write requests issued by the host computer, those write requests whose issue time occurs prior to a criterion in time are all reflected in storage areas, while those write requests whose issue time is at or subsequent to the criterion in time are all not reflected in storage areas. It is accordingly possible to prevent intermediate results of transactions from being left uncompleted.